| Marine medium&high-speed high-power diesel engines are widely used in military and civil fields.With the rising power and strengthening degree of high-speed diesel engines,the thermal load and mechanical load on the pistons of diesel engines are also increasing,so it is very important to effectively strengthen the cooling of the pistons.The commonly used intensified cooling method for combined pistons of marine medium and high speed diesel engines is pressure oil supply oscillating cooling with a double-gallery structure At present,scholars at home and abroad have relatively little research on it,and the oscillating cooling mechanism and its influencing factors are not completely clear.In this paper,a combination of visualization experimental research and numerical simulation research is used to conduct an in-depth study on the oscillating flow pattern,flow&heat transfer characteristics,and influencing factors of the combined piston oscillating cooling process of the marine medium&high-speed diesel engine.In addition,we also proposed an optimization and improvement plan for the structure of the oil cooling gallery.The main research work of this paper is as follows:According to the structure of real marine medium-speed diesel engine,a visual test bench for piston oscillation cooling was designed and built,which restored the reciprocating motion law of combined piston in diesel engine and the oscillating motion form of oil in cooling oil gallery.Based on the test bench,the visual experimental study on the oscillating cooling process in the combined piston with pressure oil supply was carried out,and the macroscopic oscillating flow characteristics such as piston oscillating flow pattern,gasliquid two-phase flow pattern and liquid phase area ratio in the oil gallery were explored under different engine speeds,different oil supply pressures and different oil supply temperatures.Aiming at a specific combined piston with double oil cooling galleries,we explored the results of multiphase flow model,turbulent flow model and wall function model on the numerical simulation.Finally,we found that CLSVOF multiphase flow model,RNGk-εturbulence model and ‘Enhanced Wall Treatment’ wall function model have great adaptability in the numerical simulation of oscillatory cooling,and it has can be in good agreement with the visualization test results.The flow characteristics and heat transfer characteristics of piston oscillation cooling of a marine medium-speed diesel engine under calibration conditions were studied by numerical simulation method,and the variation laws of oil mass flow rate at the inlet of cooling oil gallery,oil filling rate,near-wall fluid temperature and surface heat transfer coefficient with crankshaft angle were obtained.Then,the influence of key factors such as engine speed,oil inlet pressure and oil inlet temperature on the flow characteristics and heat transfer characteristics of combined piston oscillating cooling is discussed.The key indicators of flow characteristics include inlet oil mass flow rate,oil filling rate and turbulence intensity,and the key indicators of heat transfer characteristics include fluid temperature near the wall of oil gallery and surface heat transfer coefficient.In view of the structural deficiencies of the combined piston oil cooling gallery,several improved structural schemes are put forward.Finally,it is found that the improved scheme of adding two transverse oil unloading passages between the outer gallery and the inner gallery and expanding the cross-sectional area of the inlet pipe to twice has a very good effect on improving the oil circulation and the overall heat transfer capacity in the oil cooling gallery,in which the overall heat transfer coefficients of the outer gallery and the upper part of the inner gallery are increased by 21.19% and 42.63%. |
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